A great amount of effort has been expended heretofore in attempting to encapsulate or coat materials with polymer. One field of endeavor has been to encapsulate finely divided materials such as metals, metal oxides, pigments, fillers, and inorganic and organic particulates in general having a mean particle diameter of generally 2 microns or less. Many assertions have been made heretofore that the individual pigment particles have been coated with polymers, such as thermoplastic addition polymers, in order to prevent the particles from agglomerating and ultimately settling out of dispersions containing them. Additionally, attempts were made to encapsulate pigment particles for the purpose of improving paint properties including stability, gloss, hiding power, etc. The coated or alleged encapsulated particles, in many instances, were disclosed as useful in a wide variety of applications such as in the pigmented paint field as mentioned above or as electroconductive additives to plastics and other materials, toners for use in electrophotographic uses as well as many other applications.
Newman in U.S. Pat. No. 3,133,893 discloses pigment particles coated with a polymer which is polymerized in situ by stabilizing pigment particles of less than 1.5 microns, mostly between 0.01 and 0.5 microns, in an aqueous medium in which an emulsion polymerization is carried out to form the polymer coating. The dispersion is formed with a suitable dispersant which may be cationic, anionic or nonionic. The patentee states at column 3, line 45: "In general, the type of dispersant is immaterial." Newman discloses stabilization of the pigment against agglomeration by means of the polymer coating on the individual pigment particles.
Osmond, et al., in U.S. Pat. No. 3,544,500 disclose a complex process for encapsulating solid particles, e.g., pigments. The process involves first adsorbing a polymer on the surface of the particles and providing a stabilizer having an anchor group which becomes associated with the adsorbed polymer on the surface and a pendant hydrophilic component solvated by the aqueous phase and so provides a stabilizing sheath around the particles. The pretreated particles can then be subjected to aqueous emulsion polymerization of a monomer in an aqueous phase to effect the encapsulation. The polymer to be adsorbed on the surface is soluble in an aqueous medium containing the particles and a modification is effected to make the polymer insoluble. The patentees disclose at column 3, lines 67 et seq., that the polymer to be adsorbed should contain strong polar groups to promote adsorption on the surface of the particles. At column 2, lines 1, et seq., the patentees state:
"The particles may be dispersed in a solution of the polymer and stabilizer in the liquid but preferably they are dispersed in a solution of the polymer in the liquid, the stabilizer being added after dispersion of the particles. In the second stage of the preparatory treatment the nature of the solution is modified so that the adsorbed polymer is no longer soluble in the liquid continuous phase but the chain-like hydrophilic component of the stabilizing compound remains solvated by it. As a result of the modification of the solution the anchor component of the stabilizer becomes associated with the now insoluble polymer adsorbed on the disperse particles whilst the other chain-like component remains solvated by the liquid forming the continuous phase and provides a stabilizing sheath around the particles. The stabilizer may be added before the modification of the solution or at the same time as the modification takes place." PA1 "In general, because of the need to modify the solution containing the stabilizer, the stabilizer will be a non-ionic one providing a steric stabilizing sheath around the particles. Optionally such nonionic stabilizers may be supplemented by ionic types . . . and these ionic types are preferably added after that [preparatory]stage."
The patentees further state at column 3, lines 31, et seq., that:
Clarke, et al., in U.S. Pat. No. 3,580,880 disclose allegedly stable dispersions and the preparation of same. Particulate solids are said to be dispersed in a liquid with a polymer comprising one chain-like lipophilic pendant component which is capable of being solvated in a lipophilic liquid and at least one nonionic chain-like component which is solvated by a hydrophilic liquid to effect what the patentees refer to as an entropic stabilization (Column 2, lines 15 to 33). The polymer may comprise as the hydrophilic component non-ionic chain-like components with a molecular weight of 350-5000 and the lipophilic component is sufficiently long to provide a barrier 12 Angstroms in thickness around the particles.
Schofield in U.S. Pat. No. 4,349,389 describes dispersible inorganic pigment compositions in which the surface of the finely divided inorganic pigment particles is coated with a substance rendering the surface hydrophobic ("hydrophobing agent"), said substance preferably being an amphiphile containing one or more hydrophobic chains of carbon atoms interrupted by O, N or S atoms, and one or more hydrophilic groups such as amino groups, quaternary ammonium groups, carboxylic groups, etc. wherein the hydrophilic group or groups anchors the hydrophobic chain to the pigment surface and a layer of a thermoplastic polymer evenly covers and adheres to the hydrophobic chain.
Solc in U.S. Pat. No. 4,421,660 discloses aqueous dispersions of polymer coated particles, including titanium dioxide pigment particles, produced by first emulsifying polymerizable monomers with a chemical dispersant and/or surfactant and then polymerizing the monomer and coating the particles with the polymer resulting from the polymerization. The patentee discloses the use of high shear conditions, e.g., produced by an ultrasonic probe, to effect a colloidal dispersion of particles. Typically, the dispersion of solid particles is permitted to stand overnight after which the top layer of colloidal dispersed pigment is decanted and used in an emulsion polymerization. The encapsulated particles are particularly intended for applications requiring a colloidal size particulate having magnetic properties. The percent solids contents of the encapsulated particles set forth in the examples were too low without concentration for practical use in the manufacture of water-based paint.
Inspection of a photomicrograph (Sample No. 0) of allegedly encapsulated pigment presented by Solc during the prosecution of the patent application reveals titanium dioxide particles of about 0.2 to 0.3 micron in diameter with a much greater number of polymer spheres of about 0.03 to 0.05 micron in diameter. A portion of these polymer particles appears to be stuck to the perimeter of the titanium dioxide particles. The appearance of the coating suggests that Solc conducted an emulsion polymerization to produce very fine polymer particles and some of these polymer particles contacted and stuck to the surface of the titanium dioxide particles, i.e., no selective polymer growth appears to have occurred at the titanium dioxide surface. The use of sonication and very large amounts of surfactant which readily forms micelles are expected to promote the formation of very small emulsion polymerization particles of the type depicted by Solc.
Farrar, et al., in U.S. Pat. No. 4,507,422 describe aqueous dispersions of pigments containing as a dispersing agent a water soluble polymer having a polydispersity of below 1.5 (i.e., a narrow molecular weight range) and a molecular weight in the range of about 1000 to 5000. The preferred polymers are polymers of acrylic acid (or copolymers) with 2-acrylamido-2-methyl propane.
Martin in U.S. Pat. No. 4,608,401 describes finely divided water insoluble particles free of ionic charges and states that the particles are given a generally uniform polymeric encapsulation. The patent states that such materials are obtained by mixing in an aqueous reaction medium a water insoluble monomer polymerizable to form a generally water insoluble polymer free of ionic charges in the presence of a nonionic surface active stabilizing agent, thereafter polymerizing the monomer using a redox polymerization initiating system which is free of ionic groups and does not decompose to release ionic groups in the reaction medium. The patent also states that naturally agglomerated particulate materials are effectively dispersed in situ during polymerization thereby eliminating the need for preliminary grinding and/or dispersion treatments. The patentee emphasizes that the medium be substantially free from anionic or cationic surface active or dispersing agent. The non-ionic stabilizing agent is said to have sufficient hydrophilic-lipophilic power to possess good emulsifying action and have an HLB of at least about 13. The patentee broadly discloses alkylphenyl and derivatives thereof as lipophilic components and ethylene oxide chains as hydrophilic groups, i.e., from about 10 to up to 200 or more --(CH.sub.2 CH.sub.2 O)-- groups. Nonylphenol ethoxylates are the only agents used in the examples.
Solc in U.S. Pat. No. 4,680,200 describes the encapsulation of organic pigment solids in a hydrophobic polymer made by dispersing the monomer in an aqueous colloidal dispersion of the organic particles followed by emulsion polymerization.
Gibson, et al., in U.S. Pat. No. 4,413,073 disclose stable aqueous film-forming dispersions. The dispersions are said to be stable to harsh processing such as ball-milling and sand grinding which are sometimes necessary for the incorporation of certain pigments. See column 6, lines 8 to 18. The patentees do not disclose encapsulating particulate solids. Rather, the disclosure pertains to dispersions of particles of film-forming polymer such as alkyd resin in the presence of a stabilizing compound having an HLB value of at least 8 and whose lipophilic portion comprises at least one ethylenic double bond.
Daniel, et al., in European Patent Application 38730 disclose fine iron oxide (Fe.sub.3 O.sub.4) particles (less than 0.01 microns) emulsified in an anionic agent (e.g., (C.sub.12 H.sub.25 OSO.sub.3 Na) and non-ionic surfactant, e.g., ethoxylated adduct of nonylphenol having 30 ethylene oxide units.
Kitahara, et al., in U.S. Pat. No. 4,749,506 disclose a water-in-oil microgel dispersion in which a surface active agent is adsorbed or adhered onto a gel consisting of a fine particle substance and a polymer. The microgel is water-insoluble but is capable of being dispersed in both aqueous and oily solvents. The disclosed surface active agents are ethoxylated adducts of phenol derivatives wherein the ethylene oxide chain is 1 to 20 units in length.
Hoy, et al., in U.S. Pat. No. 4,426,485 disclose thickening agents comprising water-soluble, thermoplastic, organic polymer having segments of bunched monovalent hydrophobic groups. These thickening agents can find application to alter wetting characteristics of particulate materials and aid in suspending particulate materials in aqueous solutions.
Daubach, et al., in West German patent application 2,745,872, disclose dispersing agents for dyes and fluorescent brighteners which agents have the structure EQU (ROCH.sub.2).sub.2 HCO--(CH.sub.2 CH.sub.2 O).sub.n --SO.sub.3 M.sub.m H.sub.m-1
where R is phenyl, C.sub.1-18 alkylphenyl, di-C.sub.1-8 -alkylphenyl, tri-C.sub.1-4 -alkylphenyl, mono-, bis-, or tris (alpha-phenylethyl)phenyl, M is alkali metal cation or ammonium ion, n is 75 to 700 and m is 0 or 1.
Advantageously, processes for the polymeric encapsulation of finely-divided particles, e.g., pigments, are accomplished in a cost effective, efficient and environmentally-acceptable manner. Accordingly, aqueous media are the desired menstrua in which to conduct the polymerization process to encapsulate the particles. Polymerization from aqueous phases can result in the formation of precipitated polymer separate from the particles intended to be encapsulated. Thus, these new polymer particles represent a loss of efficiency in the encapsulation process. Hence, processes are sought which tend to minimize or reduce the creation or nucleation of new polymer particles. Moreover, the encapsulation process should be relatively non-complex in order to enhance its attractiveness for commercial operations. Unfortunately, the heretofore disclosed processes for encapsulation of particles in aqueous media frequently tended to have one or more shortcomings, e.g., complexity in that several preteatment steps were required to provide particles suitable for encapsulation, and even then excessive creation or nucleation of new polymer particles occurred reducing the efficiency of the encapsulation.